1. Field of the Invention
The present invention relates to externally emulsified low-solvent aqueous binders, processes for their preparation and use, as well as to coating compositions, adhesives, and the like comprising such binders.
2. Description of Related Art
Water-dilutable binders obtained by mixing or partial condensation of a polycarboxyl component which is soluble in water following neutralization of the carboxyl groups with a resinous polyhydroxyl component which per se is not dilutable in water and featuring favorable viscosity characteristics on dilution with water are described in a series of patents; for example in Austrian Patent B 328 587, Austrian Patent B 379 607, Austrian Patent B 388 738 and Austrian Patent B 388 382.
Austrian Patent B 328 587 specifies acrylate copolymers as a usable polycarboxyl component; however, on the basis of the customary monomer composition described therein, it is impossible to formulate any coating materials which would be useful in practice, since these coating materials have disadvantages in their application properties, such as the tendency toward blistering and toward running during stoving operations, even at low film thicknesses. German Patent DE-C 43 15 593 describes externally emulsified two-component coating compositions comprising a water-insoluble, hydroxyl-containing polyester or polyacrylate resin which is emulsified by adding anionic, cationic or nonionic emulsifiers in a proportion of from 0.1 to 1% relative to the mass of the resin dissolved in an organic solvent, and comprising a liquid organic polyisocyanate.
In general, externally emulsified aqueous binder systems, including the system described in DE-C 43 15 593, exhibit considerable disadvantages which considerably restrict the scope for their application. For example, they possess inadequate shear stability; the preparation of stable pigment pastes is difficult if not impossible; the resistance of the coating films produced with them is inadequate, especially to water; and, finally, their stability on storage is inadequate. "Externally emulsified" means that a substance which itself is not water-emulsifiable has been carried into an aqueous emulsion by virtue of addition of an emulsifying agent. In contrast to that, "internally emulsified" means that a substance which in itself is not water-soluble or water-dispersible has been modified by chemical incorporation of moieties that have hydrophilic character, thereby rendering the (modified) substance water-dispersible or water-emulsifiable.